Nowadays, most of the GaN-based light-emitting diodes have an epitaxial layer grown on a sapphire substrate. Sapphire has a low electrical conductivity, and the GaN-based light-emitting devices normally have a lateral structure in which both electrodes are on the same side of the device. The lateral current paths through the n-GaN layer are different in length, resulting in current blocking and decreased light-emitting device reliability. Sapphire also has a low thermal conductivity, which may limit the light-emission power and efficiency of the GaN-based light-emitting devices. Consequently, problems concerning heat dissipation, light emission and static electricity reducing can be solved by removing the sapphire substrate and forming the light-emitting devices with a vertical structure.
In a vertical light-emitting diode, an n-type conductive layer serves as an upper light-emitting surface; and a p-type conductive layer servers as a lower light-emitting surface and is welded on a heat dissipation substrate through conductive metal. The p-type conductive layer normally has a small thickness, approximately 200-800 nm; hence the distance between the active layer formed on the p-type conductive layer and the conductive metal on the heat dissipation substrate is small. The heat dissipation substrate, in some cases, may even be a metal substrate, e.g., Mo, Cu or WCu. Therefore, for example, in the process of chip dicing, generated metal particles may be splashed to the sidewall of the quantum well light-emitting layer, causing electricity leakage and ineffectiveness of the light-emitting diode; in the process of packaging, the conductive adhesive may extend to the light-emitting layer, causing electricity leakage and ineffectiveness of the light-emitting diode; and in the process of chip handling, the light-emitting layer may be damaged, also causing electricity leakage. In order to prevent the vertical light-emitting diode from the problems above to avoid short circuit, insulating materials such as SiO2 are used in the prior art to protect the sidewall of the light-emitting layer.
FIG. 1a and FIG. 1b illustrate a conventional vertical light-emitting diode structure. A heat dissipation substrate 10 is formed on a first electrode at the bottom, and the heat dissipation substrate 10 can be made of any one of the materials of: GaAs, Ge, Si, Mo, Cu, and WCu. A second electrode 12 is formed on the heat dissipation substrate 10. Welding metal 13 is formed on the second electrode 12. A third electrode 14 is formed at a central region on the welding metal 13. A semiconductor light-emitting layer 20 is formed on the third electrode 14, and the semiconductor light-emitting layer includes an n-type conductive layer made of GaN, an active layer made of InGaN, a p-type limiting layer made of InGaN or AlGaN, and a p-type conductive layer made of GaN. A fourth electrode 15 is formed on the semiconductor light-emitting layer 20. An electrical insulating layer 16 is formed on the edges of the upper surface of the semiconductor light-emitting layer 20 and extends to its sidewall; the electrical insulating layer can be made of any one of the materials of: SiO2, SiN and Al2O3. The vertical light-emitting diode in the prior art is characterized in that: short circuit protection is based on the electrical insulating material e.g. SiO2, SiN or Al2O3; however it has the drawbacks that: the insulating film of e.g. SiO2 is normally formed on the surface and sidewall of the semiconductor light-emitting layer by plasma-enhanced chemical vapor deposition (PECVD) at a certain temperature, due to the large stress inside the deposited SiO2 film and its poor adhesiveness to the semiconductor light-emitting layer, the SiO2 film may easily split or fall off, failing the purpose of short circuit protection.